The invention relates to a self-cleaning centrifuge drum for the periodical removal of a portion of the solids separated from a liquid and collecting in the peripheral part of the interior of the drum. Such a drum is known, for example, from British Pat. No. 373,117.
In this known drum, the lower portion of the drum is provided in the plane of its greatest inside diameter with a plurality of openings distributed about its circumference, which are periodically opened by an axially displaceable annular operating piston which externally envelops the lower portion of the drum. In its axial movements it is guided on the inside against the cylindrical outer surface of the lower part of the drum and on the outside on the cylindrical inside surface of a housing partially surrounding the drum and rotating with it. The housing is of bowl-shaped construction, and its free margin extends axially slightly above the openings disposed in the lower part of the drum. The drum has two control chambers for the actuation of the operating piston, each provided with one emptying bore and the control liquid being fed to them alternately through a three-way valve.
The operating piston has on its outside two annular flanges which are inclined toward one another and terminate at a distance apart from one another. In this manner an annular pocket is formed which opens toward the outside. Between the flanges, openings are created in the operating piston whose number is the same as that of the openings disposed in the lower part of the drum and which are in radial alignment with the latter.
When the upper control chamber is filled with control liquid, the operating piston moves to its lower end position. In this state, the openings in the operating piston and in the periphery of the lower part of the drum are in line with one another, so that the pocket, which acts as a lock, is in communication with the interior of the drum. In this position the pocket is closed on the outside, because the upper annular flange of the operating piston reaches into the bowl-shaped housing surrounding the drum and lies against its inner margin. The pocket fills with solids, which very soon displace inwardly most of the accompanying liquid, since they are in the area of greatest centrifugal force.
When the lower control chamber is filled with control liquid, the operating piston moves to its upper end position, whereupon the lock is separated from the inner chamber of the drum, and when the separation has been accomplished the annular pocket is opened at its periphery. The solids in the pocket are let out in this manner as through a lock.
In the known drum, it is a disadvantage that the operating piston and the housing serving for its outer guidance are on the outside of the drum periphery. They are thus located in the area of greatest centrifugal force, so that even slight irregularities will decidedly interfere with the smooth running of the drum. Even the air friction is considerably greater than it is on drums with a plain periphery. But the greatest disadvantage lies in the fact that both the operating piston and the housing partially enveloping it are of bowl-like construction and cannot be supported at the outside. It is known that a rotating hollow cylinder expands three times as much as a disk of the same diameter rotating at the same rotatory speed. For this reason, the transition between the cylindrical drum periphery and the discoidal bottom of the drum constitutes the most critical zone.
Due to the fact that, in the known drum, the free upper margin of the operating piston and the free upper margin of the housing rotating with the drum are able to expand freely under the action of centrifugal force, a perfect sealing of the interior chamber of the drum cannot be achieved. This may also be the reason why the drum disclosed by the British Pat. No. 373,117 has never been adopted in practice.